The invention concerns a glass or glass-ceramic body with a coefficient of thermal expansion below about 40.times. 10.sup.-7 /.degree.C., and having an adherent, high gloss enamel fired on its surface. The invention further concerns a system of enamel fluxes particularly adapted to producing the enamel coated article.
Heat resistant, borosilicate glassware, having a relatively low thermal coefficient of expansion on the order of 30-40.times. 10.sup.-7 /.degree. C., was developed and introduced commercially about 1915. See U.S. Pat. Nos. 1,304,622-3 granted May 27, 1919 to E. C. Sullivan and W. C. Taylor. Borosilicate glasses have found particular application in laboratory ware and baking ware, and such glassware is known and used worldwide today.
In spite of the long and widespread popularity such glassware has enjoyed, no satisfactory enamel has been available for firing on borosilicate glass surfaces. Accordingly, borosilicate baking ware has been marketed as a clear glass product, that is undecorated, over the years. Where markings became absolutely essential on borosilicate glassware, for example on measures or volumetric ware, efforts have been made to develop ion exchange stains as a color medium.
Previous difficulties in decorating borosilicate ovenware centered about an inability to fire a chemically durable enamel on the surface of an article without distorting the article shape. For example, a widely used borosilicate glass of commerce, Code 7740 glass from Corning Glass Works, has a thermal coefficient of expansion of 32.5.times. 10.sup.-7 /.degree. C. This renders the glass resistant to thermal shock, but limits the choice of compatible enamel fluxes. Further, this glass has a strain point of 510.degree. C. Consequently, warpage tends to occur if an article molded from the glass is maintained at a temperature about 660.degree. C. for any length of time. Therefore, it has proven extremely difficult to develop a chemically durable enamel flux that can be properly fired on such glassware.
The chemical durability of a glass or glass-ceramic surface is usually considered in terms of material loss per unit surface on exposure to a certain environment, e.g. water, or a specified acid or alkaline solution. However, in the case of glassware used in food preparation, e.g. baking ware, the more important consideration usually is toxic metal release. Of particular concern are lead and cadmium metal release values, these toxic metals being present in many decorating enamels as vitreous constituents and/or pigment additions.
In recognition of the potential danger from excessive toxic metal release, the Food and Drug Administration (FDA), in its Compliance Guidance Manual issued June 13, 1974, has established maximum limits which lead and/or cadmium release from an enameled surface must not exceed. In the prescribed FDA test, an enameled surface is exposed for 24 hours to 4% acetic acid at room temperature (22.degree..+-.20.degree.C.). A sample of the acid solution is then tested for absorbance in an atomic absorption spectroscope and the observed value converted to a metal concentration value on a standard curve, the metal being reported in parts per million (PPM). The reported value is based on the inside volume of a hollow article having an enamel coated or decorated inner surface and filled to a specified level with acetic acid for the test. In order to comply with FDA requirements, lead release from a food contacting surface, that is, for example, the inside of a dish in which food is prepared, served, or stored, must not exceed 7 parts per million (ppm) and cadmium release correspondingly must not exceed 0.5 ppm.
A similar test has been devised for use on exterior surfaces of a vessel or dish. Here, however, the maximum limits of metal release are somewhat higher, since there normally is no direct exposure of such surface during baking or other food preparation. Thus, the permitted metal release limits for exterior surfaces, in terms of micrograms/cm..sup.2 are 50 units of lead and 5 units of cadmium. It is of course desirable to employ enamels with much lower release values at any time during product life.
While the FDA standards are based on acid reaction on a freshly produced article, it is well known that alkaline solutions may be even more detrimental to a glass-or glass ceramic surface. Accordingly, a test has been devised in which weighed and measured samples of enameled glass are immersed in a 0.3% by weight aqueous solution of an alkaline detergent marketed by Economics Laboratories, St. Paul, Minn., under the mark Super Soilax. The solution is maintained at 95.degree. C. for 24 hours, after which the samples are removed, rinsed, dried, and weight loss determined. The loss may be based on the enamel, per se, or may be reported as lead and/or cadmium release values for comparison with FDA standards. The time of 24 hours represents an accelerated equivalent to the anticipated exposure of a dish to such conditions during its expected lifetime.
Presently, glass-ceramic materials, that is, materials developed by nucleated internal crystallization of certain glasses, are of great interest. See U.S. Pat. No. 2,920,971, granted Jan. 12, 1960 to S. D. Stookey, for a full discussion of glass-ceramic materials and their production. In particular, certain Li.sub.2 O--Al.sub.2 O.sub.3 --SiO.sub.2 glass-ceramics, having a stoichiometry such that a beta-spodumene crystal phase is developed, have a very low coefficient of thermal expansion on the order of 10 to 15.times. 10.sup.-7 /.degree.C. Accordingly, this type of glass-ceramic has found wide application in cooking ware and rangetops.
The decoration of such glass-ceramic articles is much less of a problem because the ware can be fired at considerably higher temperatures without danger of distortion. However, such higher firing temperatures involve markedly increased energy consumption. Accordingly, a drive has been under way, recently, to minimize, as far as possible, the firing temperature required in providing a high gloss, low toxic metal release enamel on glass-ceramic ware.